The invention relates in general to data transmission systems, and, more particularly, to an interface circuit for coupling transceivers to power distribution lines.
Electric power distribution networks are widely used throughout the world. It is well known that such networks are used for supplying power, but they may also provide a medium for transmitting data or other information. Such a medium is particularly advantageous for communicating with users located in remote areas. Since many of these users may already have appliances and instruments connected to a power distribution line, the costs of installing a dedicated line for conveying information may therefore be avoided.
This type of data transmission, commonly referred to as xe2x80x9cconveyed waves,xe2x80x9d takes advantage of the fact that electric power is distributed with a well established frequency. That is, the transmission and reception of signals typically involves modulating the signal to be sent to a carrier frequency unoccupied by other signals. Therefore, it is possible to send on a single line (e.g., a power line) numerous signals on non-overlapping frequency bands, and the desired signal band may be separately selected during the reception phase.
The transmission of information on power distribution lines is well suited for sending control signals to a particular site for the appliances installed at that site, which avoids the need for telephone lines or radio transmissions. Yet, transmission or reception of data signals on a power distribution line requires a suitable interface circuit between the line and the transceiver to efficiently inject a modulated carrier in the power distribution line. One of the problems associated with transmitting information on power lines is that the impedance of the medium of transmission is not constant, and the variations may be of several orders of magnitude depending on the presence or absence of loads connected to the power line.
In order to reduce the problems associated with variable line impedance, the document DH028 issued by ENEL in September 1992 states that in Italy the control of the level of the signal output by a transceiver coupled to a power distribution line should be implemented in a current mode when the impedance is below a certain value (i.e., 5xcexa9) and in a voltage when the impedance is above that value. Current interface circuits can either control the level in current mode or in voltage mode, as described in U.S. Pat. No. 4,636,771, and therefore they must be chosen as a function of the prevailing line impedance condition.
There is a need for an interface circuit that monitors the impedance of the line and switches automatically from a current mode to a voltage mode of controlling the signal to be conveyed on a power distribution line and vice-versa, depending on the actual impedance of the line.
It is an object of the present invention to provide a new method and related circuit for controlling the level of the output signal produced by a transceiver of digital data coupled to a power distribution line capable of switching automatically from a current mode to a voltage mode and vice-versa, depending on a detected impedance of the distribution line.
This and other objects, features, and advantages in accordance with the present invention are provided by a method for controlling the level of a signal produced by a transceiver of digital data coupled to a power distribution line during a transmission phase. The method may include comparing the current level of the signal with a predetermined minimum value and a predetermined maximum value, reducing the current level when it exceeds the maximum value by reducing a current gain, and switching to a voltage control mode when the current level becomes lower than the minimum value and until the current level exceeds the minimum value. Switching to the voltage control mode may include comparing the voltage level (Vref) of the signal with high and low predetermined thresholds and regulating a voltage gain.
A coupling interface for a transceiver of digital information coupled to a power distribution line for controlling the level of a signal transmitted on the line during a transmission phase is also provided. The coupling interface may include a voltage amplifier having a gain controlled as a function of a digital control datum or signal and having an input coupled to the signal to be transmitted and a current amplifier coupled to the output of the voltage amplifier delivering the signal to the power distribution line. The coupling interface may also include a first pair of comparators for comparing the voltage on a current sensing resistor in series with the output of the current amplifier with a first pair of predetermined high and low thresholds and producing a first pair of first and second logic signals, a second pair of comparators comparing a signal representing the output voltage of the current amplifier with a second pair of predetermined high and low thresholds and producing a second pair of third and fourth logic signals, and a control logic circuit receiving as an input the first and second pairs of logic signals and outputting the digital control signal.
The digital control signal may be set to reduce the gain of the voltage amplifier if the first logic signal assumes a false logic value. Further, the digital control signal may be set to maintain the gain if the first logic signal and the second logic signal assume a true logic value. Also, the digital control signal may increase, decrease, or maintain the gain when the second logic signal assumes a false logic value, if the fourth logic signal or the third logic signal assumes a false logic value, or the third logic signal and the fourth logic signal assume a true logic value, respectively. A true logic value is assumed by the first logic signal and by the third logic signal when the relevant signal does not exceed the high threshold, while a true logic value is assumed by the second logic signal and by the fourth logic signal when the relevant signal exceeds the lower threshold.